engleski

Response of rhizosphere microbial communities to legume inoculation

Responsible nitrogen use in agriculture is key to sustainability. Economic and environmental considerations dictate that we must exploit biological alternatives that can reduce nitrogen fertilization. The proportion of biologically fixed nitrogen (N) can be significantly enhanced by legume inoculation with efficient rhizobial strains. Consequently, nitrogen fertilization is reduced and, simultaneously, a successful and cost-effective legume production is ensured. However, current knowledge of the relationship between nitrogen fixation efficiency and soil microbial community structure and activity is insufficient, particularly as regards communities involved in the N cycling processes in the soil, such as nitrification and denitrification. It is assumed that rhizobial symbiotic efficiency does not only affect the ability to fix nitrogen, but has a direct impact on plants and, through root secretions, also on the abundance and dynamics of microbial populations involved in the N cycling in the rhizosphere. The main objective of this study was to evaluate the influence of inoculation with different indigenous Sinorhizobium meliloti strains on N cycle processes in the rhizosphere of alfalfa and on plant growth promotion, as well as their competitive abilities in the soil. For this purpose, a pot experiment was carried out under controlled conditions using two factors (3 x 3) on the basis of a completely randomized block design with four replications. The factors in the experiment were indigenous S. meliloti strains exhibiting different efficiency and three different alfalfa development stages. Quantitative real-time PCR assays were performed to measure the abundances of microbial populations involved in the N cycling. The inoculation effects were also examined on ammonium and nitrate contents in the rhizosphere, as well as carbon and nitrogen content in the soil, dry matter and green mass yield of the plants, chlorophyll content and nitrogen and carbon contents in the above the ground portion of the plant. The competitive ability of both S. meliloti strains used for inoculation was determined by checking their presence in the nodules of alfalfa. According to our hypothesis, the inoculation of alfalfa seeds with different S. meliloti strains altered not only the abundance of nifH genes in the rhizosphere, but also the abundance of further functional genes of microbes involved in nitrogen cycling. Similarly, the positive effect of inoculation with a highly effective strain was confirmed in most of the investigated parameters related to the plant. The plant development stages had, however, in all cases, a greater impact on the gene abundance patterns than the different treatments, as well as on most of the studied parameters of the plant itself.

nitrogen turnover; legumes; N fixation; rhizosphere; real-time PCR

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